Polyimide substrates having copper components in the form of films, coatings, lines, and pads attached to a least one surface thereof are widely used in the microelectronics industry. For example, such polyimide substrates are used in polyimide flexible circuits, printed circuit boards, and chip carriers. In another application, polyimide substrates having a copper film attached to at least one surface thereof are used to form photocells. In many of these applications, the resulting products are subjected to adverse environmental conditions that promote detachment of the copper component from the surface of the polyimide substrate. For example, the products may be subjected to environmental testing at high heat and humidity or used in outdoor service. Thus, a number of procedures have been developed to improve the adhesion of copper components to polyimide substrates.
For example, in forming circuitized polyimide substrates, a layer of chromium or titanium is first deposited onto the polyimide substrate to form a tie coat. Then a copper layer is deposited onto the tie coat, typically by a vapor deposition process such as sputtering or evaporation. The thickness of the copper layer can be increased by electroplating of copper onto the vapor-deposited layer. Thereafter, portions of the copper layer are removed by etching to form the desired circuit pattern. Unfortunately, it is also necessary to remove the tie coat by a second etch step. This second etch step is time-consuming and adds to the cost of forming the circuitized product. In addition, the waste that results from this second etch step is environmentally undesirable. Thus, additional care and cost is required to handle the waste.
Attempts have also been made to improve adhesion between copper components and polymer substrates, such as polyimide substrates, by mechanically or chemically roughening the surface of the substrate prior to deposition of a copper thereon. It is believed that the roughening improves adhesion of the copper to the polymer by mechanical interlocking. Unfortunately, such adhesion is not very durable especially when the copper-coated polymeric product is subjected to the high humidity and temperature of stress testing.
Finally, attempts have also been made to enhance adhesion by employing a polymeric adhesive layer between the copper component and the polyimide substrate. However, removal of the adhesive in selected areas of the resulting product can pose problems.
Accordingly, it is desirable to have a new method for forming adherent metal components, particularly copper components, on a polyimide substrate.